Stainless steels are generally more difficult to machine than carbon and low-alloy steels because of their high strength and high work hardening rate compared to carbon and low-alloy steels. Stainless steels require higher powered machines and a lower machining speed than carbon and low-alloy steels. Further, the high strength and high work hardening rate of stainless steels often shortens the useful tool life during machining. The aforementioned limitations, as well as several precautionary procedures for machining martensitic, ferritic, austenitic, and precipitation hardening stainless steels, are well known and are discussed in the Metals Handbook Desk Edition, pp. 15-8, 15-9 (Boyer and Gall ed. 1985).
Some grades of stainless steel have been modified by additions of elements such as sulfur, selenium, phosphorus, or lead to improve their machinability. For example, AISI Type 416, a free-machining, martensitic grade of stainless steel consists essentially of, in weight percent:
______________________________________ C 0.15 max. Mn 1.25 max. Si 1.00 max. P 0.060 max. S 0.15 min. Cr 12.00-14.00 Fe Bal. ______________________________________ In Type 416 stainless steel, carbon is present to provide the desired strength level; sulfur is present to provide good machinability; and, chromium is present for corrosion resistance.
Attempts have been made to improve the machinability of Type 416 by including manganese or a combination of tellurium, aluminum, and copper. While these elements are known to benefit the machinability of stainless steel, they are also known to detract from such desirable properties such as corrosion resistance and processability, i.e. hot workability and ease of melting, when present in too great amounts. For example, tellurium adversely affects hot workability. Too much manganese adversely affects corrosion resistance. Alloys containing aluminum often require processing by more expensive melting techniques to prevent the formation of aluminum oxide which is detrimental to tool life. Although copper is beneficial to machining in drilling, it has been discovered by the inventor that copper reduces machinability in turning unless carefully balanced with carbon and nitrogen.
Thus, it would be highly desirable to have a stainless steel alloy which has better machinability in both turning and drilling than Type 416 stainless steel and which provides at least the same level of processability, corrosion resistance and hardness capability as Type 416 stainless steel.